The control of human globin gene expression in erythroid cells involves trans-factors (substances active at distant locations in the genome), most of which have yet to be identified or clearly described. One experimental approach to their identification is to study the effects on globin gene expression of well-described trans-factors from tumor viruses. We have shown that the HTLV-l trans-factor tax1 stimulates both beta- and epsilon-promoters fused to a CAT gene, resulting in roughly 20-fold increase in CAT enzyme activity. In the case of beta-globin, only 185 bp of 5' flanking sequence is required for this effect. Also, we have shown that stimulation of the epsilon-globin promoter requires at least 114 but not more than 177 bp of 5' flanking sequence. Computer analysis reveals 2 elements in this region homologous to the tax1 response element of HTLV-I; their functional significance has been demonstrated by site directed mutagenesis. In addition, we have demonstrated sequence-specific binding of nuclear proteins from HeLa and K562 cells to one of these elements (-121/-114). Binding requires both the tax1-response-like element and an intact CACCC sequence (a cis element important to the activation of many genes) at -111/-107. Further studies will involve characterization of the tax1 induced trans-activation of globin promoters. Identification of these cis elements and the proteins that bind to them may lead to identification of proteins involved in trans-activation of globin genes by tax1. Our ultimate objective is to identify such cellular proteins that interact with tax1 to trans-activate globin genes. Study of such proteins may clarify the developmental regulation of globin gene expression in human erythroid cells.